1. Field of the Invention
The present invention relates to an electric double layer capacitor having a high withstand voltage.
2. Related Background Art
An electric double layer capacitor is a device that can store electric energy, and such a capacitor is used for a backup power source or the like that is required to be highly reliable, since the lifetime of the electric double layer capacitor is longer than that of a secondary battery. Larger electric double layer capacitors are expected to be used for power leveling or the like. However, since a typical electric double layer capacitor has a smaller energy density in a comparison with a secondary battery, the improvement of the energy density should be achieved in developing such an electric double layer capacitor.
The energy density of an electric double layer capacitor is proportional to capacitance and also proportional to a square of a withstand voltage. The withstand voltage of the electric double layer capacitor is determined depending on a withstand voltage of the applied electrolyte, i.e., a voltage just before the occurrence of electrolysis. However, for various reasons, withstand voltages of commercially available electric double layer capacitors are set to be considerably lower than the voltages at which electrolysis occurs. One of the reasons is the electrolysis of content moisture contained in the electrolyte. When the content moisture is electrolyzed, alkali is generated in the vicinity of a cathode. This alkali corrodes a sealing member, and thus, sealing performance of the product will deteriorate. JP-A-11(1999)-26329 suggests formation of a coating layer of an insulating rubber-denatured hydrocarbon resin on a surface of a sealer that is contacted with a lead at the cathode side so as to prevent deterioration of the sealing performance. However, such a coating layer is insufficient for suppressing deterioration in the sealing performance, where the deterioration is caused by alkali generated during application of a high voltage. Therefore, for securing the reliability of the product, the voltage should be lowered, and this causes a difficulty in raising the withstand voltage.
An electric double layer capacitor being used for power leveling or any other purposes relating to energy supply requires further increased capacity when compared to a circumstance that the same electric double layer capacitor is used as a backup power source. Moreover, since the operating voltage of the load is high, a plurality of electric double layer capacitors should be connected in series. When even one of the serially connected electric double layer capacitors has a problem, the capacitors of the same series cannot be used any more. Especially when a serial connection is a precondition in use, raising the withstand voltage of each capacitor is required.
A purpose of the present invention is to provide an electric double layer capacitor having a high withstand voltage.
For the above-mentioned purpose, an electric double layer capacitor of the present invention comprises a sealing member, an electrolyte and an electric double layer capacitor element, both of which are sealed in the sealing member, and the electric double layer capacitor further comprises a solid buffer to suppress fluctuation in the pH of the electrolyte. Since the solid buffer stabilizes the pH of the electrolyte, corrosion is suppressed in the electric double layer capacitor of the present invention, and thus the withstand voltage is improved. Furthermore, since the solid buffer used herein is present without being dissolved in the electrolyte, the solid buffer can be prevented from affecting characteristics other than the withstand voltage of the electric double layer capacitor.
Though there is no specific limitation on the solid buffer as long as it is remains in stable even when contacted with an electrolyte, a suitably used buffer is at least one compound selected from an oxide and a hydroxide. It is more preferable that the solid buffer is a compound (an oxide and/or a hydroxide) of at least one element selected from the group consisting of Be, Al, Si, Sc, V, Cr, Fe, Ni, Cu, Zn, Ga, Ge, Y, Zr, Mo, Ag, Cd, In, Te, La, Ce, Pr, Nd, Pm, Gd, Dy, Ho, Er, Tm, Lu, W and Pb.
It is preferable that the solid buffer adheres to a sealing member. When the sealing member comprises a case having an opening and a sealer for sealing the opening, the solid buffer can adhere to the case and/or the sealer. In an electric double layer capacitor further comprising a lead extending from the interior of the case to outside, the solid buffer can adhere to the lead in the case. In any of these preferred examples, a solid buffer suspending in the electrolyte is prevented from adhering to either a surface of an electrode or a separator composing the electric double layer capacitor element, so that lowering of the self-discharge characteristics can be avoided.
Further purposes, properties and advantages of the present invention will be described fully in the following passages. Advantages provided by the present invention are described below with a reference to the attached drawings.